The present invention relates to fuel assemblies for nuclear reactors and more particularly for pressurized water-cooled reactors. More specifically the invention relates to improvements to the guide tubes of such fuel assemblies, whose construction will be briefly described hereinafter.
For the description of fuel assemblies for known water-cooled reactors, reference can be made to French Patent Nos. 1 536 527 and 70 18 102 of the Westinghouse Electric Corporation. In elements of the type described in these patents, the nuclear fuel of the reactor is distributed in the form of a plurality of fuel rods located in the reactor core, the distribution being in the form of generally square, juxtaposed assemblies. Each fuel assembly extending over a length of several metres and having a rigid mechanical structure is supported between the upper and lower core plates. This structure has an upper end piece and a loser end piece, provided with orifices permitting the passage of the cooling water, which circulates vertically from bottom to top, and a certain number of guide tubes ensuring the cohesion of the assembly by their fixture to the upper and lower end pieces, for which they thus serve as spacers. A certain number of supporting grids arranged along each assembly and which are traversed by guide tubes and defined square cells, permit the passage and transverse supporting of the rods, which are themselves placed on the lower end piece.
In an also known manner, apart from their function of the mechanical cohesion of the assembly, said guide tubes serve to house absorbent rods used for controlling the chain reaction at any time and which are inserted as rapidly as possible into the assembly, through the upper end piece which is perforated for this purpose, when it is desired, for any random reason, to bring about a rapid shutdown of the reactor.
Conventionally, the end pieces of the fuel assemblies are made from stainless steel, i.e. a material having a relatively high neutron absorption, whereas the guide tubes are made from zircaloy having much less favourable neutron absorption properties. This difference between the materials makes it relatively difficult to solve the problems of connecting the guide tubes and the end plates.
In known constructions, the guide tube is fixed to the upper end piece of the assembly by means of an intermediate sleeve made from a material compatible with that of the end piece, because it is not possible to directly weld the zircaloy guide tube to the steel member. Thus, it is necessary to use a steel sleeve fitted on to the outer surface of the upper part of the guide tube and secured by means of a certain number of mechanical deformations ensuring the fixing of the sleeve to the guide tube. This sleeve is itself welded or brazed on the one hand to the upper end piece, and on the other hand to the upper grid, thus ensuring the mechanical connection of the end piece to the structural assembly.
However, these arrangements lead to a certain number of serious disadvantages, particularly with regards to the possibilities of the remote disassembly or detachment of the upper end piece, e.g. in a pond, for replacing a defective fuel rod.
In order to make the connections used for fitting the guide tube to the upper end piece of a fuel assembly dismantlable, various solutions equipped with screwing means are used. Although in theory such systems make it possible to effectively obtain disengageable connections, practical requirements make it very difficult to produce such solutions which are not open to criticism. Thus, a certain number of criteria, most important of which are referred to hereinafter, make the construction of such disengageable fixtures particularly difficult. Thus, to be acceptable, it is desirable for such a connection:
not to cause axial torsion or twisting of the guide tube during its screwing or unscrewing, PA1 not to require the prior angular positioning of the parts to be assembled, PA1 not lead to risk of the threads seizing, particularly during dismantling after an irradiation period, PA1 to provide a safety restraint of the system in order to provide any accidental slackening, PA1 to permit easy remote disassembly, bearing in mind the operating conditions in a storage pond, and PA1 to permit easy reintroduction of the upper end plate onto the end fittings of free guide tubes.